For the purposes of reducing a coefficient of friction and suppressing wear in various friction-sliding places, lubricating oils have been used in every industrial machine.
In general, current lubricating oils are constituted so as to form a fluid film in a sliding gap under a mild friction condition (fluid lubrication condition) and to form a semi-solid coating film at a frictional interface under a severe friction condition (boundary lubrication condition). That is, the current lubricating oils contain a low-viscosity oil (namely, a base oil) capable of revealing a low coefficient of friction and a chemical which for the purpose of preventing direct contact between interfaces to be caused after the low-viscosity base oil has been broken under a sever friction condition, is able to react with an interface thereof (for example, an iron interface) to form a tough and soft boundary lubricating film capable of imparting a low coefficient of friction. Though the chemical is dissolved in the base oil, it is accumulated with time at an interface thereof due to the reaction with an interface raw material (in general, steel). However, at the same time, the chemical also reacts with the majority of the face which is not directly related to sliding, and accumulation occurs, whereby the valuable chemical is consumed. In addition, even when the chemical is consumed, the base oil does not vanish but actually remains as various decomposition products; and in many cases, such accelerates deterioration of the lubricating oil per se. Moreover, the boundary lubricating film per se formed by the reaction of the chemical is also peeled off by friction-sliding under a severe condition, and the boundary substrate per se is also peeled off; and they are floated or deposited (sludged) together with the foregoing reaction decomposition products, thereby impairing lubricating ability of the lubricating oil and causing a factor in deteriorating its expected performance. In order to prevent this matter, in general, an antioxidant, a dispersant, a cleaning agent and the like are added to a lubricant (Patent Document 1).
In the light of the above, in the majority of current lubricating oils, for the purpose of reducing the friction under an extremely severe condition (boundary lubrication condition) and also the purposes of reducing and inhibiting side effects of the added chemical, a new chemical is further added. Moreover, for the purpose of reducing a lowering of the lubricating function to be caused due to fine worn powders formed from the interface per se by the wear and decomposition floats of the chemical, a new chemical is further added. And since functions of various chemicals are related to each other in the lubricating oil, it is inevitable and unavoidable that a period of time when the lubricating oil can function as a whole and exhibit the best lubricating effect becomes short due to exhaustion and deterioration of the respective chemicals. It may be said that this is a vicious cycle of a certain kind. In consequence, it is not easy to greatly improve the composition for the purpose of improving performances of current lubricating oils.
However, all of the foregoing compounds called “chemical” are ones containing an element reactive with the iron interface, and furthermore, substances formed through a reaction between such a compound and iron have ability to reduce friction and wear thereof. The element which is essential for the lubrication is phosphorus, sulfur or a halogen and furthermore, is zinc or molybdenum working competitively and complementarily. The former three are distinctly an environmentally hazardous element, and release thereof into the air even as an exhaust gas must be utterly avoided.
In addition, lubricating oils to be used for internal combustion engines, automatic transmissions and the like are required to be made low in viscosity for the purpose of achieving fuel saving, and at the same time, from the viewpoints of effective utilization of resources in recent years, reduction of waste oil, cost reduction of lubricating oil user and the like, a requirement for realization of long drain of a lubricating oil is increasing more and more. In particular, following high performances of internal combustion engines, high outputs, severe driving conditions and the like, lubricating oils for internal combustion engine (engine oils) are being required to have higher performances.
However, in conventional lubricating oils for internal combustion engine, in order to ensure heat or oxidation stability, it is generally conducted to use a highly refined base oil such as hydrocracked mineral oils, etc., or a high-performance base oil such as synthetic oils, etc. and blend the base oil with a sulfur-containing compound having peroxide decomposing ability such as zinc dithiophosphate (ZDTP), molybdenum dithiocarbamate (MoDTC), etc., or an ashless antioxidant such as such as phenol based or amine based antioxidants, etc. However, it may not be said that the heat or oxidation stability by itself is always sufficient. Moreover, though it is possible to improve the heat or oxidation stability to some extent by increasing the blending amount of the antioxidant, there is naturally a limit in an effect for enhancing the heat or oxidation stability according to this technique.
And from the viewpoint of an environmental issue such as a reduction of emission of carbon dioxide, etc., the engine oils are required to be reduced in the content of sulfur or phosphorus for the purposes of enhancing fuel-saving performance and durability and keeping catalytic ability for cleaning an exhaust gas. On the other hand, in diesel engines in recent years, though an emission control mechanism of particulate matter, such as a diesel particulate filter (DPF), etc., is started to be installed, diesel engine oils are required to realize a low ash from the standpoint of an issue of plugging of the mechanism. The realization of a low ash of engine oils means a reduction of a metallic cleaning agent, and it is an extremely important problem to ensure diesel engine cleaning properties to be kept by blending a large amount of a metallic cleaning agent or an ashless dispersant, in particular, cleaning properties of a top ring groove with a high heat load.
When an internal combustion engine is taken as an example, the foregoing lubrication is concerned with lubrication of portions other than a combustion chamber and a lubricating composition. However, as for the lubrication of the combustion chamber, there is actually a big problem, too. That is, studies for controlling (preventing or decreasing) a reduction of deposits formed in a fuel introducing port of the combustion chamber, or a reduction of friction and wear to be caused thereby, by trace additives to be added to the fuel have been continued over a period of many years.
In particular, in recent years, from the viewpoint of exhaust gas regulation, it has been becoming essential to realize a low sulfur concentration of a fuel composition. However, there is a concern that according to this, the lubricating properties are lowered, thereby causing a lowering of durability of a valve gear mechanism including cams and valves. Here, it is also driven by necessity to review the conventional element contributing to a reduction of friction and wear.
That is, in order to exhibit efficacy by small amount addition, reactivity with an interface raw material is an essential requirement, and nevertheless an element capable of revealing desired low friction by forming a boundary lubricating film is essential, at the same time, it is required to reduce sulfur, phosphorus and heavy metals, the presence per se of which is problematic. The lubricating oils are a material supporting the current industrial machines themselves, and even if they are not easily displaced, this is the moment at which a composition of lubricating oil and a lubrication mechanism per se as a background thereof must be seriously reviewed by the latest scientific technologies and functional raw material technologies after a lapse of 150 years or more.
At the beginning, while it has been described that “For the purposes of reducing a coefficient of friction and suppressing wear in various friction-sliding places, lubricating oils have been used in every industrial machine”, a mission of the lubricating oil itself is to keep and preserve a motor function of machine. Though we make a machine work and utilize it, when the work (action) is taken out (counteraction), friction is inevitably caused at a mutually sliding interface. In order to reduce vigorous wear generated by the friction and prevent a mechanical damage such as seizure, etc. from occurring, it is necessary to ensure a sliding gap, and for that reason, various solid or liquid lubricating films have been applied.
A theoretical analysis of the behavior of such a liquid film in the friction state starts from the matter that the Navier-Stokes equations describing the motion of a viscous fluid in the hydrodynamics were applied to a gap with a narrow Reynolds. In those days, an experimentally verified phenomenon in which a wedge-shaped oil film in a bearing generates a high hydrodynamic pressure was theoretically explained, thereby laying the foundation of the fluid lubrication theory of the day.
According to this theory, in view of the fact that the Sommerfeld number which is utilized as a basic characteristic number of the bearing design is expressed by the following equation, it is noted that a film thickness d of a sliding gap is related to a pressure P, a viscosity η (→also correlated with a temperature T) and a sliding velocity V. Since the film thickness d itself of the sliding gap accurately depends upon an average roughness Ra of the surface thereof, it may be said that factors relating to breakage of the film thickness d of the sliding gap are the pressure P, the temperature T, the viscosity η, the average roughness Ra of the surface and the sliding velocity V.Sommerfeld number S=[η(T)*R(bearing radius)*V(velocity)]/[2πP(pressure)*d2(gap)]  [Numerical Formula 1]
From the viewpoint of keeping the oil film, as for the factors influencing the gap d, it may be easily analogized that at a high temperature, factors of a reduction of the viscosity of the oil film and an interface roughness are important and that under a high pressure, the pressure and the pressure dependency of the oil film viscosity are naturally important.
In consequence, the history of a technology for keeping a liquid film started from control of the viscosity of a base oil. First of all, in order to prevent breakage, an oil with relatively high viscosity, namely a highly viscous oil is used. However, a machine must start up, and at that time, a high viscosity is disadvantageous. Furthermore, in general, at the start-up time, the temperature is lower than that at the operation time, in most cases, the oil hardly moves because of its extremely high viscosity; and therefore, in a sense of utterly avoiding breakage at the high-temperature time, a high viscosity index oil which is originally low in viscosity was used, and furthermore, a polymer (viscosity index improver) was added to a low-viscosity base oil.
The technology developed in response to severer conditions at a high temperature and under a high pressure is a technology concerning an interface protective film (boundary lubricating film) capable of firmly adhering directly to an interface, in particular an iron interface and having flexibility. Historically, starting from the addition of a soap, inorganic films such as iron chloride, iron sulfide, iron phosphate, etc. were formed; and in recent years, reactive and low-friction organometallic complexes such as Mo-DTC, Zn-DTP, etc. have been developed, and a trace amount thereof is added to a base oil.
Though there were an improvement of viscosity physical properties against the temperature as described previously and a technical development of forming a lubricating film by another method, a technical and simple approach as in the invention, in which a viscosity-pressure modulus is controlled and optimized for the purpose of inhibiting breakage of an oil film while controlling the viscosity against the pressure has not been revealed yet.
However, the theory concerning the viscosity-pressure modulus has been surely established with the times.
As for the friction mechanism, there is known an elastic fluid lubrication mechanism between the foregoing mild fluid lubrication mechanism and severe boundary lubrication mechanism. A theoretical study of this elastic fluid lubrication mechanism started from the study regarding the true contact face shape and the generated pressure, published by Hertz in 1882; established by a summary of the EHL elastic fluid lubrication theory by Petrosevich in 1951; and became a practical theory by an oil film formation theory taking into consideration of elastic deformation by Dowson/Higginson in 1968.
A region where this elastic fluid lubrication mechanism works is a friction region under a high pressure of, for example, several tons per cm2, namely about several hundred MPa. At a glance, though such a condition is severe, in fact, since iron starts to cause elastic deformation within such a pressure range, the area of the true contact face of the iron interface coming into contact with the oil film increases, and the substantial pressure becomes low. That is, within this region, so far as an elastic limit of iron or oil film breakage is not caused, the coefficient of friction does not increases, and it may be said that such a region is a “blessed region” for the sliding interface. Moreover, at the same time, in this region, an oil film made of a general lubricating oil such as mineral oils becomes high in viscosity by about 1,000 times that at the time of atmospheric pressure, but there may be the case where it becomes low in viscosity by only about 500 times depending upon a chemical structure of the raw material. Barus expressed this phenomenon relative to pressure dependency of the viscosity of liquid in terms of the following equation (VII) and exhibited that an increase rate a of viscosity which is inherent in the substance to pressure is related (Non-Patent Document 1).η=η0 exp(αP)  (VII)
Here, α represents a viscosity-pressure modulus; and η0 represents a viscosity at atmospheric pressure.
Moreover, Doolittle advocated a thought of a free volume model that a viscosity of liquid is determined by a ratio of an occupied volume of molecule occupied in a liquid volume and a free volume generated by thermal expansion (Non-Patent Document 2).η=A exp(BV0/Vf)  (VIII)
Here, η represents a viscosity; V0 represents an occupied volume of molecule; and Vf represents a free volume.
In comparison between the equation (VIII) of Doolittle and the equation (VII) of Barus, it is noted that the viscosity-pressure modulus α is in inverse proportion to the free volume of molecule. That is, what the viscosity-pressure modulus is small suggests that the free volume of molecule is large. In consequence, it is noted that it is possible to control the pressure dependency of the viscosity of liquid by optimizing a chemical structure of raw material, namely, it is possible to provide a raw material having a lower viscosity than oils constituting current lubricating oils under the same high-load and high-pressure conditions by optimizing the chemical structure. For example, assuming that an oil film of a true contact part is formed by a raw material having a viscosity-pressure modulus α of about a half of that of mineral oils or hydrocarbon based chemical synthetic oils such as poly-α-olefins, which are usually used as a lubricating oil, this elastic fluid lubrication region is laid under a milder condition. That is, in usual lubricating oils, even under a high load which is classified into the boundary lubrication region, in view of the fact that a cooling effect by an oil film as well as low pressure and low viscosity of the true contact site is added due to the elastic deformation of the interface and the low-viscosity oil film under a high pressure, it is expected to substantially avoid the boundary lubrication region and realize an ideal lubrication mechanism made of only fluid lubrication.
In recent years, it is disclosed that discotic compounds having a plurality of radially arranged relatively long carbon chains and lubricating oils containing the same (namely, a metallic raw material-free lubricating oil) exhibit a low coefficient of friction in the elastic fluid lubrication region (for example, Patent Documents 2 to 4). Such a discotic compound has a discotic core and side chains radially extending from the discotic core, and it is expected that a sector-shaped free volume can be inevitably ensured in a highly arranged state, too. In consequence, discotic or tabular compounds having radially arranged side chains have many free volumes in common as compared with an occupied volume thereof, and therefore, they exhibit a small viscosity-pressure modulus. That is, it is expected that the viscosity is relatively small even under a high pressure, and lower viscosity and lower friction properties are revealed under a high pressure (Non-Patent Document 3).
However, what is common among these raw materials is the matter that the viscosity thereof is larger by one digit than that of mineral oils and chemical synthetic oils usually used for lubricating oils, and it is absolutely impossible to use a large amount of such a raw material inexpensively in place of low-viscosity base oils.
That is, though the viscosity under a high pressure is defined by the viscosity η0 and the viscosity-pressure modulus α as expressed by the foregoing equation (VII), when a low-viscosity base oil is actually used, it already starts to be broken in an elastic fluid lubrication region, and it becomes in a viscosity-free state, namely an elasto-plastic body under a high pressure. It has been elucidated that easiness of breakage of this lubricating oil film is correlated with an agglomerated state of fluid molecules, namely a packing state of lubricating oil molecules and can be evaluated by a product αP of the viscosity-pressure modulus α and the pressure P (Non-patent Document 4).
In general, the lubricating oil film acts as a viscous fluid when the product αP is not more than 13, as a visco-elastic fluid when the product αP is between 13 and 25 and as an elasto-plastic body when the product αP is 25 or more, respectively. In the case where two kinds of lubricating oil films having the same viscosity η under a certain pressure P, where a viscosity-pressure modulus is defined as α1 and α2, respectively, and also a normal pressure viscosity is defined as η1 and η2, respectively, the following equation is established.Inη=Inη1+α1·P=Inη2+α2·P 
In the case of 18=α1·P<α2·P=24, namely α1/α2=18/24, it is noted that when the pressure P is increased a little more, the film having a viscosity-pressure modulus α2 becomes an elasto-plastic body and is more easily broken even under the same pressure at the same viscosity.
In consequence, even when a base oil having a relatively large η0 to such extent that it can be used even in a fluid lubrication region is utilized, since the viscosity-pressure modulus α of an chain hydrocarbon such as mineral oils constituting a base oil is large, there is eventually a tendency that the viscosity η under a high pressure becomes large, and it has been considered that neither base oil having a visco-elastic fluid region nor organic compound, each of which has a low η0 capable of imparting a low coefficient of friction under fluid lubrication and a low α capable of imparting a low coefficient of friction under elastic fluid lubrication at the same time, is present so far.
For the time being, even if a raw material capable of clearing the foregoing restrictions could be developed, taking into consideration necessary conditions of base oils requiring large-amount feed and low costs, it is difficult to provide a raw material satisfying all of them. Therefore, as for engine oils which are essential to be low in viscosity for the purpose of achieving low fuel consumption, it may be considered that there is a background wherein a concept itself for effectively utilizing elastic fluid lubrication was not recognized. It may be said that convergence of the raw material development to a combination of a current low-viscosity based oil and a trace chemical capable of forming a boundary lubricating film as described at the beginning was an inevitable result.
Patent Documents
    [Patent Document 1] JP-T-2005-516110    [Patent Document 2] JP-A-2006-328127    [Patent Document 3] JP-A-2007-92055    [Patent Document 4] JP-A-2006-257383Non-Patent Documents    [Non-Patent Document 1] C. Barus, Am. J. Sci., 45 (1893), page 87    [Non-Patent Document 2] A. K. Doolittle, J. Appl. Phys., 22 (1951), 1471    [Non-Patent Document 3] Masanori HAMAGUCHI, Nobuyoshi OHNO, Kenji TATEISHI and Ken KAWATA, Preprint of the International Tribology Conference (Tokyo, 2005-11), page 175    [Non-Patent Document 4] Nobuyoshi OHNO, Noriyuki KUWANO and Fujio HIRANO, Junkatsu (Lubrication), 33, 12 (1988), 922; 929